REFRACTORIES 493 Plastic Refractory Clays.óClays combining good plasticity with high refractoriness are not of common occurrence. While there are a number of such deposits in the United States, the majority of these materials show plasticity at the expense of heat-resisting power, that is, they soften at temperatures below that of cone 32. Occasionally they develop in limited quantities as the result of the weathering of flint clay. The most important sources of such clays are to be found in the so-called ball clays of Paris and Whitlock, Tenn., Mayfield, Ky., Anna, 111., Enid and Senatobia, Miss., and in the plastic fireclays from St. Louis and southern Ohio. These clays are of great importance as a source of clay for the manufacture of refractories for the glass industry and of graphite crucibles. Plastic clays of good quality are also found in the Golden district, Colorado, Utah and California. Analyses of several typical clays of this class are given in table: TABLE 2 Titan Silica Alumina Ferric oxide ium oxide Lime Magnesia Potash Soda Loss on ignition St Louis ............... 54.64 28 75 3 00 1 85 0 30 0 35 0 15 11 00 Whitlock, Tenn., Clay No. 5 47.70 33.28 1.21 not det. 0.45 0.75 not det. not 16.25 det. Mayfield, Ky., Clay No. 4. 53.22 32.36 1.17 not det. 0.40 0.83 not det. not 12.44 det. Mayfield, Ky., Clay No. G. 60.80 24.62 0.88 2.80 0.46 0.13 0.50 0.38 10.36 Klingenberg, Germany ..... 50.76 29.26 1.51 1.61 1,08 0.73 0.85 0.19 14.24 The chemical composition of the German Klingenberg clay has been given for comparison inasmuch as it represents one of the best known bonding clays known, especially for the manufacture of graphite crucibles. It will be noted that these compositions show a higher silica and lower alumina content than the kaolins and flint clays, and in addition show replacement of the AUOs and combined water by fluxing constituents, iron oxide, lime, magnesia and alkalis. This replacement when continued beyond the bounds indicated results in further decrease of the heat-resisting qualities until we come to the plastic fireclays of low grade. This makes it necessary to classify the plastic fireclays of this type according to their softening point into No. 1 materials softening at or above cone No. 32, No. 2 clays softening between cones No. 30 to 32, No. 3 clays softening between cones 28 and 26 and No. 4 materials fusing between Nos. 26 and 19. This classification is an arbitrary one and does not hold strictly for siliceous clays as will be shown later. A large class of fireclays associated with the coal measures of Pennsylvania, Maryland, Ohio, Kentucky, West Virginia and Indiana belong to the types softening between cones 28 to 32. These are useful for bonding purposes in the manufacture of firebricks and shapes. Such clays are available in enormous quantities in conjunction of the coals of these states. All of the plastic bond clays, whether of the No. 1 grade or not, fire to a buff color which may become more and more discolored through the presence of iron pyrites or iron oxide as we descend in the scale of refractoriness. The purest as well as the strongest clays are probably the so-called ball clays, the plasticity and bonding power of which is reinforced by the presence of organic matter. This type of clay seems to be preferred in the manufacture of graphite crucibles, and other special refractories.